1. Field of the Invention
The present invention relates to image processing apparatuses and methods, and in particular, to image processing apparatus and method in which, when the color gamut (hereinafter referred to as the "gamut") of an output system is narrower than the gamut of an input system, colors close to colors obtained by the input system can be outputted by compressing the gamut of color signals outside the gamut of the output system in the direction along which the value of a color-difference formula decreases to its minimum.
2. Description of the Related Art
Recently, with the price reduction and speed increase of electronic devices, desktop publishing (DTP) has become widespread, and various electronic devices for processing color images are used.
In many cases, such electronic devices have different gamuts (color-reproduction ranges) which can be expressed in accordance with the types thereof. Thus, the concept of "device independent color" for expressing a predetermined image with identical colors by a plurality of electronic devices is on the point of being introduced.
The concept of device independent color is realized as a color management system (CMS).
As shown in FIG. 12, when color signals from a predetermined input system (video camera 61, scanner 62, monitor 63 or the like) are converted to color signals for a predetermined output system (monitor 63, printer 64 or the like), the color signals from the input system are temporarily converted (profiled) to signals in a color space (CIE (commission internationale de l'eclairage)/XYZ, CIE/L*a*b*, etc.) which does not depend on a device by using a predetermined conversion formula or conversion table (profile), and the converted color signals are converted to the color signals for the output system.
For example, when the monitor is used as an input system, and the printer is used as an output system, as shown in FIG. 13, red, green and blue signals corresponding to the monitor are initially converted to color signals CIE/XYZ or CIE/L*a*b*, which do not depend on the device by using a profile corresponding to the monitor. The converted color signals which do not depend on the device are converted to printer color signals CMY by using the profile corresponding to the printer.
Concerning methods for profiling, a method using a physical model like a conversion matrix, a method using a lookup table (LUT), a method using a neutral network, and so forth have been devised.
In the above manner the CMS converts color signals from a predetermined input system to color signals for a predetermined output system. When the gamut of the input system is broader than the gamut of the output system, it is impossible for the CMS to convert in identical colors the color signals from the predetermined input system to the signals for the predetermined output system.
For example, the monitor performs color reproduction with an additive mixture of colors by causing three-primary-color (red, green and blue) phosphors to produce colors. The printer performs color reproduction at a predetermined gradation on a recording medium such as paper by a subtractive mixture of colors with four types of ink: cyan, magenta, yellow and black. As shown in FIG. 14, the gamut of the printer 64 is narrower than (different from) the gamut of the monitor 63. Consequently, in the figure the colors in hatched ranges on the monitor 63 cannot be reproduced by the printer 64.
Accordingly, in such a case, color signals from the monitor are converted to signals within the gamut of the printer by performing color-gamut mapping (hereinafter referred to as "gamut mapping"), with the original image information (gradation, tones, and so forth) maintained as much as possible.
The gamut mapping is performed in a color space which does not depend on a device, and is normally performed, particularly in a CIE/L*C*h color space (color space formed by converting an L*a*b* color space to polar coordinates) corresponding to human visual characteristics. In the CIE/L*C*h, L*, C* and h represent lightness, chroma and hue, respectively.
Conventionally, when gamut mapping is performed, hue is fixed in a CIE/L*C*h color space, and lightness and chroma are compressed on a two-dimensional plane comprising lightness and chroma.
For example, there are a number of methods: a method (as shown in FIG. 15) decreasing (compressing) only chroma while fixing lightness and chroma; a method (as shown in FIG. 16) determining the lightness and chroma of output color signals so that the sum of the lightness-difference square and the chroma-difference square in input color signals and output color signals is minimized while fixing hue.
According to the above-described method in which gamut mapping is performed while fixing hue, lightness or chroma is greatly compressed, problems occur because compression of lightness decreases contrast and a stereoscopic effect of an image, and a deterioration in chroma decreases brightness to form an image from which an impression cannot be made.
Therefore, when the above-described color-gamut mapping is performed with respect to an image which has very high chroma and is stereoscopic such as a CG (computer graphics) image, considerable characteristics of the image are lost.